Arc discharge lamps require a ballast for operation. The ballast supplies the requisite open circuit voltage to start and maintain an arc in the discharge tube as well as limiting the current through the discharge tube. One type of ballast uses a high voltage pulse to initiate breakdown in the discharge tube. Arc tube breakdown is the first phase of lamp starting and is therefore essential for lamp operation. The typical high voltage pulse for a ballast of this type has an amplitude between three and four kilovolts (Kv) with a pulse width of 1.0 .mu.s at 2.7 Kv. There are two commercial ballast methods for applying the typical voltage to the lamp. The first method applies the pulse voltage to the center contact of the lamp base; and the second method divides the pulse between the center contact and the shell of the base. The second method, referred to as the split lead design, has an unusual characteristic, floating the lamp lead wires such that both lamp wires carry pulse voltage with respect to ground. When the pulse voltage is applied to the lamp, 1.7 Kv is applied to the center contact of the lamp and an opposite potential of approximately equal magnitude is applied to the shell of the lamp base.
There is now available a relatively new type of ceramic arc tube that utilizes a design that contains essentially three distinct sections. See, for example, U.S. Pat. Nos. 4,795,943 and 5,424,609. See also, Attorney Docket Nos. 96-1-213 and 97-1-009, filed Oct. 2, 1998 and incorporated herein by reference. The three sections are: the main, central body or arc chamber where the discharge takes place and two legs, one on either side of the body, which contain the electrode structure and the lead-ins therefor. The electrode structure comprises an external lead, an internal lead and an electrode. The internal lead connects the external lead to the electrode that is located within the arc chamber. The arc chamber, of course, also houses the arc generating and sustaining medium. The arc chamber, and thus the medium, continues into each of the opposed legs that contain the electrode structure.
One of the characteristic advantages of the preformed and presized ceramic arc tubes over their quartz predecessors is the consistent lamp to lamp geometry. This geometric uniformity results in consistent heat transfer mechanisms and consistent radiation from the arc tube. This consistency greatly enhances lamp performance. Such lamps are observed to have minimum lamp to lamp variations of color temperature, lumen output and color rendering index.
It is often necessary to use a glow bottle in addition to a ballast that supplies high voltage to start discharge lamps. These glow bottles comprise a hermetically sealed capsule, usually of quartz, which contain a partial pressure (i.e., &lt;1 atmosphere) of argon, nitrogen or other gas mixtures. They may additionally contain a partial pressure of mercury. These glow bottles contain an additional lead-in that facilitates the "glow" or ionization of their contained gases when a sufficient potential is applied to the glow bottle lead-in. The glass vessel of the glow bottle must be in close proximity to a lead-in of the opposite potential for the ionization of the enclosed gas to occur. Upon energization of the glow bottle, UV is generated, which UV initiates the arc discharge in the lamp. Such glow bottles are shown in U.S. Pat. No. 4,818,915.
The use of glow bottles, while effective, adds to the cost of the lamp and, furthermore, is generally not possible to use with a ceramic arc tube. Such ceramic arc tubes are usually encased in an aluminosilicate outer jacket that closely surrounds the arc tube leaving insufficient room to allow adequate placement of the glow bottle. Also, since the aluminosilicate outer jacket is an effective absorber of UV radiation, it is not effective to place a glow bottle outside of the jacket.
Further, since the environment between the inside of the outer jacket and the arc tube must be a vacuum when a ceramic arc tube is employed, it is not possible to use that environment as a source of UV radiation to enhance starting.
Other methods that are being employed facilitate lamp starting use hazardous materials such as radioactive krypton 85.